The present invention relates to rotary vacuum machines comprising a primary pump having complementary profiles or implementing volume transfer.
A drawback of such rotary vacuum machines with a primary pump having complementary profiles is that they produce discharge noise that can lead to inconvenience and discomfort when they are in use.
This discharge noise is due to the pressure difference between the inlet and the outlet of the atmospheric stage or outlet stage of the primary pump. Because of this pressure difference, and because the primary pump having complementary profiles acts at each stage by transferring volume and not by applying compression, shockwaves are produced when the low pressure volume transferred by the atmospheric stage finds itself suddenly exposed to the atmosphere. The outside gas at atmospheric pressure enters into the volume at high speed prior to being subsequently discharged by the pump. The opposition to movement of the two very fast gas flows gives rise to a shockwave which gives rise to loud bangs.
The phenomenon grows with increasing pressure difference between the inlet and the outlet of the pump, i.e. for example, during continuous operation with the vacuum machine maintaining a high vacuum inside an enclosure.
Attempts have been made to reduce discharge noise by putting a non-return valve in the discharge orifice of the atmospheric stage. Such a non-return valve closes and thus limits noise transmission when the discharge rate of the pump is low. The effectiveness of that technique is nevertheless insufficient, particularly when the pump needs to extract a non-negligible flow rate of gas, e.g. during treatment steps in the manufacture of semiconductors in a vacuum enclosure where the vacuum is created by the vacuum machine.
Proposals have also been made to reduce the discharge noise of rotary vacuum machines by adding a static attenuator with internal chambers and baffles to the discharge outlet. Nevertheless, that technique is not suitable for variations in the rate at which gas is discharged by the pump, and it presents risks of dead zones in the attenuator becoming clogged in the event of any back flow of gas suitable for producing a deposit.
It might also be thought that discharge noise could be reduced by designing a primary pump in which the atmospheric stage gives rise only to a very small drop in pressure between its inlet and its outlet. But that would require an additional stage to be added to the primary pump, which is of no advantage in obtaining and maintaining low pressure in the vacuum enclosure controlled by the vacuum machine. The only advantage of this additional stage is to reduce discharge noise, yet this additional stage is a structure that is complex and expensive since it needs to be made with the same precision qualities as a normal pump stage in the making and assembly of the complementary profiles of the two rotors that rotate relative to each other.
The problem proposed by the present invention is that of designing a new discharge noise attenuator structure for rotary vacuum machines having complementary profiles that provides effective suppression of the audible effect of discharge shockwaves, and that presents a structure that is simple, reliable, and inexpensive, having no complementary profiles or systems requiring synchronization.
The invention also seeks to provide such an attenuator which adapts effectively to variations in the rate at which gas is discharged by the pump, while also avoiding any risk of clogging.
To achieve these objects, and others, the invention provides an attenuator of discharge noise from rotary vacuum machines having a primary pump with complementary profiles, the attenuator comprising, interposed between the discharge from the primary pump and the outlet to the atmosphere, at least one transfer device having independent cavities which move sequentially between the discharge from the pump and the outlet to the atmosphere, being successively in communication with the outlet to the atmosphere, then isolated, then in communication with the discharge from the pump, then isolated, and then again in communication with the outlet to the atmosphere, and so on, so as to transfer the volume of gas discharged by the pump from the pump discharge to the outlet to the atmosphere while continuously isolating the pump discharge from the outlet to the atmosphere.
In a preferred embodiment, the cavities are made in at least one rotor rotating in a chamber of a stator having an inlet orifice putting one or more cavities into communication with the pump discharge, and an outlet orifice putting one or more other cavities into communication with the outlet to the atmosphere.
In a practical embodiment, the rotor is a disk having peripheral cavities isolated from one another and coming sequentially into register with the outlet orifice, with a solid portion of the wall of the chamber of the stator, with the outlet orifice, with another solid portion of the wall of the chamber of the stator, and again with the outlet orifice, and so on.
Preferably, the other solid portion of the wall of the chamber of the stator flares progressively so as to provide a progressive leakage gap which increases on approaching the outlet orifice. As a result, the progressive leak enables the volume of the cavity to bring its pressure slowly into equilibrium with the atmosphere by throttling the high pressure gas, the pressure already being in equilibrium when the cavity travels past the outlet orifice, thereby further reducing discharge noise.
In an advantageous embodiment, applicable to primary pumps having two coupled parallel rotors, the discharge noise attenuator comprises two rotors with parallel shafts rotating in two respective chambers of the stator and connected in parallel between a common inlet orifice and at least one outlet orifice.
In an improved embodiment, the discharge noise attenuator further comprises a bypass circuit with a non-return valve, the bypass circuit putting the inlet orifice directly into communication with the outlet orifice to the atmosphere whenever the gas pressure in the inlet orifice exceeds atmospheric pressure by a predefined pressure threshold. As a result, without reducing the efficiency of the primary pump, the attenuator makes it possible to discharge the high gas flow rate delivered by the primary pump while it is establishing a vacuum in a treatment enclosure. This enables the attenuator to be dimensioned so as to be just sufficient for evacuating the gas flow during stages of operation under steady conditions in which a vacuum is being maintained by the vacuum machine, with the bypass circuit having a non-return valve enabling surplus gas flow to pass through during transient stages in which the gas flow rate is much higher than that which can be discharged by an attenuator of such dimensions.
In a discharge noise attenuator of the invention, the cavity transfer device can be driven by the rotary vacuum machine to which it is mechanically coupled, or by an auxiliary motor. It can be placed adjacent to the discharge from the vacuum machine, or at a distance therefrom, at the outlet from a connection pipe.
The invention also provides a vacuum machine whose discharge is connected to the atmosphere via such a discharge noise attenuator as defined above.